Singletons/Autoload scripts are basically nodes that are children of the root and the same level in the hierarchy as the current scene node. They are referenced by the node's name which doesn't necessarily have to be the script's name and the variables can be accessed directly through that.
e.g. in the autoload tab
name: the_autoload path: res://my_script.gd
my_script.gd which is the autoload script.
var my_var = 5
Another script in the scene.